The most appropriate doc right now is the User's Guide, though I'm going to write a better one on the Wiki sometime soon.

Long story short, if you really want to play with it, currently you need to set PYTHON_TARGETS and USE_PYTHON to matching versions. Then, PYTHON_SINGLE_TARGET to one of the versions in PYTHON_TARGETS.

USE_PYTHON is used for old Python packages. PYTHON_TARGETS are used by new packages when the package can be made to work with more than one installed Python version. PYTHON_SINGLE_TARGET is used when the package can only work with one Python implementation of choice.

Since I don't want to play with it at all, can I just remove all those?
I assume sensible defaults will apply..._________________"The problem with quotes on the internet is that it is difficult
to determine whether or not they are genuine." -- Abraham Lincoln